It is proposed to use an animal model to learn about the relationship of situs inversus viscerum to congenital heart malformations. Work in this laboratory has shown that mice homozygous for a gene (iv) causing situs inversus hava a relatively high incidence of heart malformations and other abnormalities which show a striking resemblance to human patients with a set of syndromes characterized by heterotaxia and heart malformations including the polysplenia, and asplenia syndromes and less frequently isolated dextro- and levo-cardia. Mice with this mutation will be used in the following studies: 1. Examination of embryos and fetuses at various stages of development in order to learn about the morphogenetic steps giving rise to these malformations. 2. Investigation of the effects of changes in genetic background on the expressivity of the situs inversus gene causing heart malformations. 3. Investigation of the very early stages of heart development to learn more about the process of formation of normal and abnormal D and L cardiac loops.